Calculating the Astronomical Unit (AU), Astronomers observed the Venus transit from various places. Because of the parallax effect, observers like A and B saw Venus's path shifted on the Sun's face. By trigonometry, this shift & the distance between the observers you can determine the distance to Venus. It's about 0.35 AU, so knowing the distance in miles, you can get the length of the AU in miles. Mona Evans, "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/article...

"Black drop effect" observed in transit of Venus by Captain Cook and Charles Green. Observers found it hard to work out when the disk of Venus was just fully within the disk of the Sun. This meant the timings varied considerably from observer to observer. (Armagh Observatory) ©Mona Evans, ©Mona Evans, “Transit of Venus – Captain Cook 1769” http://www.bellaonline.com/articles/art28591.asp

Transit of Venus 2004 including Venus and a time lapse of the transiting International Space Station. Very few people would have been able to observe this event and the ISS took no more than a second to pass by. Amazing photo by Tomas Maruska (SAAD). ©Mona Evans, "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/articles/art28229.asp

Captain Cook memorial at Venus Point, Tahiti. He headed an expedition sent by the Royal Society of England to observe the 1769 transit of Venus. This is where they did the main observations. (Photo: Frederick Sears) ©Mona Evans, “Transit of Venus – Captain Cook 1769” http://www.bellaonline.com/articles/art28591.asp

Transit of Venus June 8, 2004. The planet almost looks like a bit of cloud, but you can find it on the right just below center. (Photo: David Cortner) ©Mona Evans, "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/articles/art28229.asp

IMAGE: Venus in sole visa (1639), the account of a transit of Venus observed by Jeremiah Horrocks. It was printed in 1662 by Polish astronomer Johannes Hevelius along with Mercurius in sole visa, his account of a transit of Mercury. "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/articles/art28229.asp

Double transit. In this photo by the brilliant astrophotographer Thierry Legaut, you can see Venus in transit on June 6, 2012. But click to enlarge and you can also see that he has caught the Hubble Space Telescope also crossing in front of the Sun. ©Mona Evans, "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/articles/art28229.asp

Transit of Venus 2012 (taken in H alpha). With this photo Chris Warren won the "Our Solar System" section of the Astronomy Photographer of the Year competition. He just caught it in a small gap in the thick cloud. ©Mona Evans, "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/articles

Transit of Venus 2012. A view of the transit across the rising Sun in Italy. (Credit: Niki Giada) ©Mona Evans, "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/articles/art28229.asp

Transit of Venus 2012. Picture of the transit taken from the Exploratorium live feed. A calcium line solar filter gives it the cool blue color. ©Mona Evans, "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/articles/art28229.asp

Transit of Venus 2012. Seen from Huntington Beach, California with Boeing 727. (Credit: Jim Nista on Flickr) ©Mona Evans, "Transit of Venus - Measuring the Solar System" http://www.bellaonline.com/articles/art28229.asp